How concrete bag yield works
A bag of concrete mix yields far less volume than its weight suggests. How yield is stated, why it varies with the mix, and how to turn a pour volume into whole bags to buy.
Last reviewed · Written and maintained by the MaterialBuyCalc editorial team
Yield is volume, weight is weight
A bag of concrete mix is sold by weight, but what you need is volume: how much space the mixed concrete fills. The relationship between the two depends on the mix design, the aggregate used and how much water the mix takes. There is no general formula from pounds to cubic feet, which is why any calculator that converts one to the other without asking is guessing.
The yield is printed on the bag
Manufacturers state it, usually as the approximate cubic feet or litres of mixed concrete one bag makes. It is the number the calculator on this site asks for, and it is the only reliable input. If you are choosing between products, compare yield per bag rather than weight per bag: they do not rank the same way.
Mixing loses material
Hand mixing leaves concrete in the barrow, on the board and in the bag. Machine mixing leaves less but still leaves some. An overage allowance covers this, and it matters more with bags than with ready-mix because the losses happen on every batch rather than once.
There is a size where bags stop making sense
Bags are excellent for post holes, small pads and repairs. For a full slab, mixing forty or fifty bags by hand is a very long day, and the result varies between batches in a way a delivered load does not. Work out the volume first: if it is approaching a cubic yard, price up ready-mix before committing to bags.